Clarification: A megapixel (shortened as MP) is the number of one million pixels. A pixel is a separate component of a digital photo. The resolution amount of a photo will be decided by the number of megapixels, so a digital photo with a higher number of megapixels has the larger resolution.
Now, because a digital camera can create a photo which contains ten million or even a higher number of pixels, “megapixel” is often used for determining the resolution levels.
Commonly, in the specification list of a digital camera, the “MP” is shortened for megapixel.
To define the sizes of photo prints depending of the amount of resolution, please see the print size vs. resolution table.
Shortened as: MP